1. Field of the Invention
The present invention relates to a method for driving a plasma display panel (PDP), and it is suitable for a surface discharge type and an AC type PDP. The surface discharge type means a structure in which display electrodes to be an anode and a cathode during display discharge for securing a luminance level (first electrodes and second electrodes) are arranged in parallel on a front or a back substrate. One of challenges for the AC type PDP is background light emission that is light emission in areas to be not lighted within a screen.
2. Description of the Prior Art
FIG. 1 shows a cell structure of a typical surface discharge type PDP. The PDP 1 comprises a pair of substrate structures (including a substrate and cell elements disposed on the substrate). The front substrate structure includes a glass substrate 11 and plural sets of display electrodes X and Y arranged on the inner surface of the glass substrate 11 in such a way that a set of display electrodes X and Y corresponds to a row of a matrix display. Each of the display electrodes X and Y includes a transparent conductive film 41 that forms a surface discharge gap and a metal film 42 overlaid on the edge portion of the transparent conductive film 41, and each of the display electrodes X and Y is coated with a dielectric layer 17 made of a low melting point glass and a protection film 18 made of magnesia. The back substrate structure includes a glass substrate 21 and address electrodes A arranged on the inner surface of the glass substrate 21 in such a way that an address electrode A corresponds to a column. The address electrode A is covered with a dielectric layer 24, on which a partition 29 is disposed for dividing a discharge space into columns. The upper face of the dielectric layer 24 and side faces of the partition 29 are covered with fluorescent material layers 28R, 28G and 28B for a color display. Italic letters (R, G and B) in FIG. 1 represent light emission colors of the fluorescent materials. The color arrangement has a pattern in which cells in a column has the same color and red, green and blue colors are repeated in turn. The fluorescent material layers 28R, 28G and 28B emit light when being excited locally by ultraviolet rays emitted by discharge gas. A structure of one column in one row corresponds to a cell, and three cells constitute one pixel of a display image. Since the cell is a binary light emission element, integral light emission quantity of each cell of each frame has to be controlled for displaying a color image.
FIG. 2 shows an example of a frame split for a color display. The color display is one type of gradation displays, and a display color is defined by combining luminance values of red, green and blue colors. The gradation display utilizes a method in which one frame includes plural subframes each of which has a luminance weight. As shown in FIG. 2, one frame includes eight subframes (SF denotes subframe in FIG. 2). The ratio of integral light emission quantities, i.e., the ratio of luminance weights of these subframes is set to 1:2:4:8:16:32:64:128 or approximate values, so that 28(=256) gradation levels can be reproduced. For example, in order to reproduce gradation level 10, cells are lighted between subframe 2 having weight 2 and subframe 4 having weight 8 and are not lighted in other subframes.
An initialization period, an address period and a sustaining period are assigned to each subframe. An initialization process is performed in the initialization period for equalizing wall voltage of all cells, and an addressing process is performed in the address period for controlling wall voltage of each cell in accordance with display data. In addition, a sustaining process is performed in the sustaining period for generating display discharge only in cells to be lighted. One frame is displayed by repeating the initialization, addressing and sustaining processes. However, each subframe usually has a unique addressing process. In addition, periods of the sustaining processes are different depending on the luminance weight. Furthermore, the initialization process can be performed not in every subframe but only in a specific subframe (e.g., in the first subframe) so that background luminance is reduced and contrast is improved.
FIG. 3 shows the conventional drive waveforms. A common waveform is applied to the address electrodes A as many as columns of the screen except the address period, while a common waveform is applied to the display electrodes X as many as the number n of rows in every period. In FIG. 3, the waveforms for the address electrode A and the display electrode X are shown by the gross. In addition, the display electrodes Y as many as the number n of rows are used as a scan electrode for selecting a row in the address period. Therefore, a common waveform is applied to these display electrodes Y except the address period in the same way as the address electrode A. FIG. 3 shows waveforms for the display electrode Y(1) of the first row and the display electrode Y(n) of the last row as representatives.
The conventional operation in the initialization period includes two stages. In the first stage, an ascending obtuse waveform pulse is applied to display electrodes Y. Obtuse waveform is a generic term used to refer to pulse waveforms having a gentle leading edge. Namely, the operation in the first stage is a bias control for increasing potential of the display electrode Y simply. On this occasion, in order to shorten the time until reaching a predetermined potential, a positive offset bias is given to the display electrode Y, and a negative offset bias is given to the display electrode X. Then in the second stage, a descending obtuse waveform pulse is applied to the display electrode Y. Namely, the bias control is performed for dropping the potential of the display electrode Y simply. In the address period, a scan pulse is applied to the display electrodes Y one by one for the row selection. In synchronization with the row selection, an address pulse is applied to the address electrodes A corresponding to cells to be lighted in the selected row. Thus, address discharge is generated and a predetermined quantity of wall charge is formed in cells to be lighted. In the sustaining period, a positive sustain pulse is applied to the display electrode Y and the display electrode X alternately. At each application, display discharge is generated between display electrodes (hereinafter referred to as an XY-interelectrode) of the cell to be lighted.
At the start time of the initialization period, i.e., at the end of the sustaining period of the preceding subframe, there are cells with relatively much wall charge remained and cells with little wall charge. A cell that was lighted correctly in the previous subframe (hereinafter referred to as a previously lighted cell) has much wall charge remained, while a cell that maintained unlighted state correctly in the previous subframe (hereinafter referred to as a previously unlighted cell) has little wall charge remained. Here, “correctly” means faithfully to display data. If the addressing process is performed in the state where the charge quantity is different between cells as mentioned above, an error is apt to occur in which address discharge is generated in a cell that is not to be lighted. The initialization is important as a preparation for enhancing reliability of the addressing.
FIG. 4 is a diagram for explaining a principle of the conventional initialization. The initialization that is explained below is an operation for equalizing wall voltage between the previously lighted cell and the previously unlighted cell and for controlling it to be a set value suitable for the addressing. As an initialization waveform, a waveform that is a combination of a positive obtuse waveform and a negative obtuse waveform is used. In order to explain the principle simply, an initialization operation limited between two electrodes α and β will be explained. The voltage that is applied to the αβ-interelectrode (i.e., between the electrode α and the electrode β) is the potential difference between the electrode α and the electrode β. In other words, it is a relative value of the potential of the electrode β to the potential of the electrode α. The above-mentioned waveform of the initialization portion shown in FIG. 3 becomes the same waveform as in FIG. 4 when taking the display electrode Y as a reference and noting the operation of either the XY-interelectrode or the AY-interelectrode.
First a descending obtuse waveform pulse having the amplitude Vr1 is applied to the αβ-interelectrode, and then an ascending obtuse waveform pulse having the amplitude Vr2 is applied to the same. The solid line indicates a variation of the voltage that is applied to the interelectrode, while the broken line and the dotted line indicate variations of the cell charge quantity (wall voltage). However, it should be noted that the wall voltage is plotted after reversing positive and negative signs. The action of applying the obtuse waveform pulse is deeply related to the cell state when the previous subframe is finished. The wall voltage when the cell was lighted in the previous subframe (hereinafter referred to as the wall voltage in the previously lighted cell) is shown in the broken line, while the wall voltage when the cell was not lighted in the previous subframe (hereinafter referred to as the wall voltage in the previously unlighted cell) is shown in the dotted line.
In the AC type PDP, since a voltage component due to electrification is added to the applied voltage component, the effective voltage that is applied to the discharge space (hereinafter referred to a cell voltage) becomes as follows.(cell voltage)=(applied voltage)+(wall voltage) 
Since the sign of the wall voltage is reversed, the level of the cell voltage at any time is indicated by the distance between the dotted line (or the broken line) and the solid line in FIG. 4. If the solid line is under the broken line (or the dotted line), the cell voltage is negative. If the solid line is above the broken line (or the dotted line), the cell voltage is positive. Therefore, the cell voltage is negative while the negative obtuse waveform pulse is applied in the first half, and the cell voltage is positive while the positive obtuse waveform pulse is applied in the second half, as shown in FIG. 4.
At the time t0 before starting the initialization, the wall voltage is negative both in the previously lighted cell and the previously unlighted cell (Since the sign is reversed, the dotted line and the broken line above the line indicating zero volt represent negative wall voltage). As illustrated, the negative wall voltage is higher in the previously lighted cell. As the negative voltage that is applied to the cells in this state is increasing gradually, the cell voltage increases. Since the previously lighted cell becomes more negatively charged, discharge starts at the time t1 in the previously lighted cell earlier than in the previously unlighted cell. Once the discharge starts, electrification of the wall charge occurs so that the cell voltage is kept at the discharge start threshold level −Vt1 in the case where the electrode α is a cathode, and wall voltage corresponding to the electrification quantity is generated (hereinafter this phenomenon is expressed as “wall voltage is written”). Discharge starts in the previously unlighted cell at the time t2 that is a short time after the start of discharge in the previously lighted cell. Once the discharge starts, wall voltage is written so that the cell voltage is kept at the threshold level −Vt1 in the previously unlighted cell, too. The application of the descending obtuse waveform pulse is finished at the time t3. At this time point, the wall voltage has the value of −Vr1+Vt1 in the previously lighted cell as well as in the previously unlighted cell.
Next, the polarity of the applied voltage is reversed, and the positive obtuse waveform pulse is applied to the αβ-interelectrode. Since the wall voltage in the previously lighted cell is made the same value as the wall voltage in the previously unlighted cell by the above-mentioned application of negative obtuse waveform pulse, discharge starts at the same time t4 in both cells. The discharge continues till the end of the positive obtuse waveform while changing the wall voltage. The cell voltage is maintained at the discharge start threshold level Vt2 in the case where the electrode α is an anode. The wall voltage is Vr2−Vt2 at the time t5 when the discharge finished. Since the threshold level Vt2 is a constant unique to the discharge between the electrodes α and β, the wall voltage after the application of the positive obtuse waveform pulse is finished depends on the amplitude Vr2 of a predetermined applied voltage.
For improving contrast of a display, it is effective to reduce light emission in the initialization, especially light emission in the previously unlighted cell. Either in a static image or in a moving image, noting a cell for displaying a black color or a dark color within a screen, the condition often occurs where the cell becomes the previously unlighted cell from a certain subframe to the following one or more subframes. Namely, supposing that in the initialization of the noted subframe the noted cell is a cell not to be lighted (unlighted cell) that is affected by the light emission in the initialization more easily than the cell to be lighted, the cell is likely to be the previously unlighted cell. Therefore, if the light emission in the previously unlighted cell is reduced, a contrast ratio can be increased. The contrast ratio is determined by total light emission quantity in the previously lighted cell and light quantity of undesirable light emission in the previously unlighted cell.
In order to secure the initialization, it is necessary to increase the amplitudes of the first and the second obtuse waveform pulse so that the written quantities of the positive and negative wall voltage are increased. However, the increase of the amplitude may increase the light quantity of the undesired light emission and may decrease the contrast ratio.
Conventionally, concerning the write quantity of the wall voltage in the previously unlighted cell, there is a problem that it is difficult to determine the optimum value that enables compatibility between performing initialization securely and reducing the background light emission. If the cell has only two electrodes, its operation is simple, so that the relationship between the applied voltage and the operation can be expected easily. In contrast, the cell has three electrodes in the practical plasma display panel, and the three electrodes influence each other resulting in a complicated operation. Therefore, the drive condition has to be optimized by trial and error. Difficulties in optimizing the write quantity of the wall voltage will be explained in detail as follows.
FIG. 5 shows the appropriate initialization in the conventional method. FIG. 6 shows the inappropriate initialization in the conventional method. In a three-electrode structure PDP, the relationship among three electrodes becomes known if two of three electrodes are analyzed. Since an actual driving process controls mainly the discharge at the XY-interelectrode and the AY-interelectrode, it is preferable to perform the analysis noting voltages at the XY interelectrode and the AY-interelectrode.
Though the applied voltage waveforms shown in FIGS. 5 and 6 do not seem to correspond to waveforms shown in FIG. 3 at first glance, they substantially correspond to one another. Even if the ascending or descending obtuse waveform pulse is applied only to the display electrode Y as shown in FIG. 3, the voltage waveform at the XY-interelectrode in the initialization period is similar to the waveform shown in FIGS. 5 and 6. In FIGS. 5 and 6, the solid line shows a variation of the applied voltage, the broken line shows a variation of the wall voltage in the previously lighted cell, and the dotted line shows a variation of the wall voltage in the previously unlighted cell. Since the wall voltage is plotted after positive and negative signs are reversed similarly to FIG. 4, the distance between the solid line and the broken line or the dotted line can be read as the cell voltage between corresponding electrodes in FIGS. 5 and 6, too.
In the discharge due to application of the obtuse waveform pulse, the discharge start threshold level is an important parameter. Therefore, the discharge start threshold level in the three-electrode structure is defined as follows.
VtXY: discharge start threshold level at the XY-interelectrode when the cell voltage at the XY-interelectrode is positive
VtYX: discharge start threshold level at the XY-interelectrode when the cell voltage at the XY-interelectrode is negative
VtAY: discharge start threshold level at the AY-interelectrode when the cell voltage at the AY-interelectrode is positive
VtYA: discharge start threshold level at the AY-interelectrode when the cell voltage at the AY-interelectrode is negative
VtAX: discharge start threshold level at the AX-interelectrode when the cell voltage at the AX-interelectrode is positive
VtXA: discharge start threshold level at the AX-interelectrode when the cell voltage at the AX-interelectrode is negative
As an example, the wall voltage at the XY-interelectrode just before the initialization is started (i.e., at the time t0) is negative in the previously lighted cell and positive in the previously unlighted cell, and the wall voltage at the AY-interelectrode is zero in the previously lighted cell and positive in the previously unlighted cell (note that positive and negative signs of the wall voltage are reversed in FIGS. 5 and 6).
In FIG. 5, when both the applied voltages (negative) at the XY-interelectrode and the AY-interelectrode increase, the cell voltage in the previously lighted cell reaches the threshold level at the time t1 first, and discharge at the XY-interelectrode starts in the previously lighted cell (hereinafter referred to as XY-discharge). This discharge lasts until the applied voltage reaches the negative peak value, so that the cell voltage at the XY-interelectrode is kept at −VtYX. Namely, the wall voltage changes in response to the variation of the applied voltage. The XY-discharge starts in the previously unlighted cell at the time t2 after the time t1. Also in the previously unlighted cell, similarly to the previously lighted cell, the discharge continues until the applied voltage reaches the negative peak value, so that the cell voltage at the XY-interelectrode is kept at −VtYX. Therefore, the wall voltage at the XY-interelectrode is −VtYX in the previously lighted cell as well as in the previously unlighted cell at the time t3 when the application of the obtuse waveform pulse in the first stage finishes.
Noting the AY-interelectrode, both in the previously lighted cell and the previously unlighted cell, the wall voltage at the AY-interelectrode varies after the XY-discharge starts. However, this variation is not caused by the discharge at the AY-interelectrode (hereinafter referred to as AY-discharge) but is a relative change in accordance with the variation of the wall voltage at the XY-interelectrode. Therefore, the cell voltage at the AY-interelectrode is not maintained at the threshold level −VtYA but continues to increase simply toward the negative side. If the amplitude of the first stage obtuse waveform pulse applied to the AY-interelectrode is not large enough, the discharge at the AY-interelectrode does not start either in the previously lighted cell or the previously unlighted cell. For this reason, at the time t3 when the first stage application of the obtuse waveform pulse is finished, the wall voltage at the AY-interelectrode in the previously lighted cell is different from that in the previously unlighted cell. The wall voltage of the previously lighted cell is larger than the wall voltage in the previously unlighted cell.
When the second stage application of the obtuse waveform pulse starts, the polarity of the applied voltage is reversed. First, the AY-discharge starts in the previously lighted cell at the time t4. During the discharge, the wall voltage at the AY-interelectrode changes so that the cell voltage in the previously lighted cell at the AY-interelectrode is kept at VtAY. Responding to this change, the cell voltage at the XY-interelectrode also changes. However, the change at the XY-interelectrode is a phenomenon that the wall voltage of the XY-interelectrode changes relatively by the discharge at the AY-interelectrode, and the wall voltage at the XY-interelectrode is not controlled directly. The direct control starts at the time t6 when the discharge at the XY-interelectrode starts.
In the previously unlighted cell, the XY-discharge starts at the time t5, and during the discharge the wall voltage of the XY-interelectrode changes so that the cell voltage at the XY-interelectrode is kept at VtXY. The wall voltage at the AY-interelectrode also changes. However, this is a phenomenon that is caused by the relative change of the wall voltage at the AY-interelectrode due to the XY-discharge and is not a phenomenon that is caused by a direct control of the wall voltage at the AY-interelectrode by the AY-discharge. The direct control starts at the time t7 when the discharge at the AY-interelectrode starts.
When the application of the obtuse waveform pulse in the second stage finishes, the wall voltage at the XY-interelectrode is VrXY2−VtXY, and the wall voltage at the AY-interelectrode is VrAY2−VtAY both in the previously lighted cell and in the previously unlighted cell. Namely, the necessary condition for controlling the wall voltage at the XY-interelectrode and the wall voltage at the AY-interelectrode to a desired value is that discharge is generated both in the XY-interelectrode and in the AY-interelectrode by the second stage application of the obtuse waveform pulse, and that the discharge periods overlap each other in time scale. Hereinafter the phenomenon that discharge is generated at two interelectrodes (at two positions) at one time is referred to as “simultaneous discharge”.
The action of the cell explained above is merely an example, and there are other examples. For example, the AY-discharge may be generated after the XY-discharge is generated in the previously lighted cell by the second stage application of the obtuse waveform pulse. In which interelectrode the discharge will be generated, the XY-interelectrode or the AY-interelectrode, depends on the state of the wall voltage just before the initialization and the set voltage of the first and the second obtuse waveform pulse. However, whichever discharge is generated first, the drive voltage has to be set so that the discharge is generated both at the XY-interelectrode and the AY-interelectrode simultaneously during the second stage application of the obtuse waveform pulse.
In FIG. 6, the light emission quantity in the previously unlighted cell is reduced by decreasing the amplitude of the first obtuse waveform pulse. However, the simultaneous discharge is not generated in the previously lighted cell during the second obtuse waveform pulse application. The wall voltage at the XY-interelectrode in the previously lighted cell when the second obtuse waveform pulse application is finished is not the target of the control. This may make the addressing of the previously lighted cell uncertain and may cause incorrect lighting or incorrect extinguish.
As explained above, it is very difficult to determine the lower limit of the wall voltage write quantity in the previously unlighted cell while controlling the complicated discharge in the three-electrode structure. Therefore, an adequate improvement of the darkroom contrast ratio in a PDP display has not been achieved. In addition, if only the improvement of the darkroom contrast ratio is regarded as important, the incorrect lighting will occur easily, resulting in significant display instability.